I like Bittrex a lot. It's fast and the interface is great.

However, Steem (and Golos, "the Russian Steem") is frequently down for maintenance. I was unable to withdraw there for the past 2 days, and though Steem came back up, Golos withdrawal has remained down.

It's more reliable than Poloniex, but the withdrawal situation is not unique to them.

Having said that, I still recommend Bittrex as it appears to be the best option for now.